getActionBar().hide();

<style name="NoActionBar" parent="@android:style/Theme.Holo.Light">
    <item name="android:windowActionBar">false</item>
    <item name="android:windowNoTitle">true</item>
</style>

<activity android:theme="@style/NoActionBar" ... />

<activity
android:theme="@android:style/Theme.NoTitleBar.Fullscreen"
....
>

<application
    .
    .
    android:theme="@android:style/Theme.Holo.Light.NoActionBar"
    . >

public class xxxActivity extends ActionBarActivity{

public class xxxActivity extends Activity {

ActionBar actionBar = getSupportActionBar();
actionBar.hide();

this.requestWindowFeature(Window.FEATURE_NO_TITLE);

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_my);
    getSupportActionBar().hide();
}

<style name="AppTheme.NoActionBar" parent="Theme.AppCompat.Light.DarkActionBar">
      <item name="windowActionBar">false</item>
      <item name="windowNoTitle">true</item>
      <item name="android:windowFullscreen">true</item>
</style>

android:theme="@style/AppTheme.NoActionBar"

<style name="NoActionBar" parent="@style/Theme.AppCompat.Light">
<item name="android:windowNoTitle">true</item>
</style>

<activity android:theme="@style/NoActionBar" ... />

<style name="AppTheme" parent="Theme.AppCompat.NoActionBar">
    <!-- Customize your theme here. -->
</style>

requestWindowFeature(Window.FEATURE_NO_TITLE);
getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, 0);

<style name="MyTheme" parent="android:Theme.Material.Light.NoActionBar">

style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar>

<style name="AppTheme" parent="AppBaseTheme">
    <item name="android:windowActionBarOverlay">true</item>
    <item name="android:actionBarStyle">@style/NoActionBarStyle</item>
    <item name="android:windowContentOverlay">@null</item>
</style>
<style name="NoActionBarStyle" parent="android:Widget.Holo.ActionBar">
    <item name="android:backgroundSplit">@null</item>
    <item name="android:displayOptions"></item>
</style>

ActionBar actionBar = getSupportActionBar();
actionBar.hide();

<activity
        android:name=".MainActivity"
        android:theme="@android:style/Theme.Holo.NoActionBar"
        android:label="@string/app_name" >

protected void onCreate(Bundle savedInstanceState) {
   super.onCreate(savedInstanceState);
   android.support.v7.app.ActionBar actionBar = getSupportActionBar();
    if (actionBar != null) {
        actionBar.hide();
    }
    setContentView(R.layout.activity_main);

requestWindowFeature(Window.FEATURE_NO_TITLE);
setContentView(R.layout.activity_main);

<!-- custom application theme. -->
    <style name="MarkitTheme" parent="Theme.AppCompat.Light.NoActionBar">
        <!-- Customize your theme here. -->
        <item name="colorPrimary">@color/colorPrimary</item>
        <item name="colorPrimaryDark">@color/colorPrimary</item>
        <item name="colorAccent">@color/colorAccent</item>

        <item name="android:windowNoTitle">true</item>
    </style>

<activity android:name=".MainActivity"
            android:theme="@style/MarkitTheme">

android:label="@string/empty_string"

<string name="empty_string">""</string>

<style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar"></style>